(1) Field of the Invention
The present invention relates to a current supply arrangement for rectifying three-phase current into multi-pulse DC current                with at least one three-phase AC current transformer with a transformer core or three single phase AC current transformers, each having a transformer core, wherein        the three-phase AC current transformer comprises three first secondary-side coils, which each are arranged on a corresponding leg of the transformer core, or each of the three single-phase AC current transformers has a first secondary-side coil, wherein each secondary side coil is arranged on a leg of the transformer cores,        with at least one first rectifier comprising first converter valves        wherein the first converter valves are connected with the first secondary-side coils to the first rectifier.        
(2) Description of Related Art
Such current supply arrangements are known from textbooks. For example, the textbook “Grundlagen der Leistungselektronik” (Fundamentals of Power Electronics), K. Heumann, ISBN 3-519-06110-4 describes a current supply arrangement, wherein secondary-side coils of a three-phase AC current transformer are connected with first converter valves to an M3-rectifier.
With the current supply arrangement described in the textbook, current from an AC circuit can be supplied to a DC load. A DC voltage can be obtained at the output of the current supply arrangement, wherein the magnitude of the DC voltage is determined essentially by the transformation ratio of the transformer. The magnitude of the DC voltage cannot be varied.
However, DC loads are known which require that DC voltages of different magnitudes are supplied in different situations. In a simple situation, a DC load is to be connected to DC voltages having two different magnitudes.
Different DC voltages at the output of an M3-rectifier can be realized by using controllable converter valves, as described in the same textbook. The controllable converter valves may be operated, for example, with phase-angle control. The voltage at the output of the rectifier can then be continuously adjusted at least over a certain range.
However, adjusting the voltage by phase-angle control has the disadvantage that due to the higher fraction of harmonics and the ensuing distortion reactive power, the power factor decreases with increasing phase angle controlling the controllable converter valves.
The invention therefore starts from the problem to improve a current supply arrangement of the aforedescribed type so that at least two output voltages can be provided with a high power factor. Another object of the invention is to provide a substantially constant DC voltage at the output of the rectifier.